Artificial implants, including hip joints, shoulder joints and knee joints, are widely used in orthopedic surgery. Artificial hip joints are generally ball and/or socket joints, designed to match as closely as possible the function of the natural joint. The ball and socket joint of the human hip connects the femur with the pelvis, wherein the ball-shaped head of the femur is positioned within a socket-shaped acetabulum of the pelvis. The head of the femur or ball fits into the acetabulum, forming a joint which allows the leg to move forward, backward and sideways in a wide range of motion. The acetabulum is lined with cartilage, which cushions the bones and allows the joint to rotate smoothly and with minimal friction. An envelope of ligaments connect the pelvis and femur, covering the joint and stabilizing it. The cartilage also renders the joint sufficiently strong to support the weight of the upper body, and sufficiently resilient to absorb the impact of exercise and activity. A healthy hip allows the leg to move freely within its range of motion, while supporting the upper body and absorbing the impact that accompanies certain activities. Various degenerative diseases and injuries may require replacement of all or a portion of a hip using synthetic materials. Prosthetic components are generally made from any or combinations of metal, ceramic, and/or plastic material.
Total hip arthroplasty and hemi-arthroplasty are two procedures well known within the medical industry for replacing all or part of a patient's hip. A total hip arthroplasty replaces both the femoral component and the acetabular surface of the joint, so that both a femoral prosthesis and an acetabular prosthesis are required. To replicate the natural kinematics of a hip joint, a total hip prosthesis has three parts: the stem, which fits into the femur and provides stability; the ball, which replaces the spherical head of the femur; and an acetabular prosthesis, which replaces the hip socket. Each part is available in various sizes in order to accommodate a range of body sizes and types. In some designs, the stem and ball are one piece; other designs are modular, allowing for additional customization in fit.
A conventional acetabular prosthesis may include a cup, a cup and a liner, or in some cases only a liner, all of which may be formed in various shapes and sizes. Generally, a metal cup and a polymeric liner are used. The liner may be made of a variety of materials, including polyethylene, ultra high molecular weight polyethylene, metal, and ceramic materials. The cup is usually of generally hemispherical or partially hemispherical in shape and features an outer surface and an inner surface that is adapted to receive a cup liner. The liner fits inside the cup and has inner and outer surfaces. The cup liner is the bearing element in this type of acetabular component assembly. The outer surface of the liner corresponds to the inner surface of the cup or acetabulum, and the liner inner surface receives the head of a femoral component. An acetabular cup may include a highly polished inner surface in order to decrease wear.
The liner inner surface can be characterized by features relative to an axis, such as an axis of rotation through the center of the inner surface. This axis may or may not be aligned with the central axis or axis of rotation of the cup. In a typical liner the inner surface has a hemispherical or partially hemispherical geometry and is also referred to as the internal diameter. In such liners, the geometry of the internal diameter can be characterized as concentric to an axis that runs through the center of the internal diameter such as the axis of rotation of the cup, outer diameter of the liner or otherwise.
An acetabular prosthesis may be fixed in the reamed acetabulum of a patient. Such a prosthesis may include a cup (or a cup and liner assembly) that is fixed by placing screws or other retaining devices through apertures in the cup, by securing the cup with cement, or by using bone ingrowth material on the outer surface of the cup. In other cases, spikes, pegs, or fins around the rim of the cup are used to help hold the implant in place until new bone forms. In some cases, only a liner is cemented in a patient due to poor bone stock. Any combination of these structures or techniques may be used.
A femoral prosthesis used in total hip arthroplasty generally includes a spherical or near-spherical head attached to an elongate stem with a neck connecting the head and stem. In use, the elongate stem is located in the intramedullary canal of the femur and the spherical or near-spherical head moves in a manner corresponding to relative motion between the pelvis and femur, (“articulates”) relative to the acetabular component. Femoral prostheses used in total hip arthroplasty procedures may or may not differ from a prosthesis used in a hemi-arthroplasty, described below. However, the femoral head of each type prosthesis is generally a standard size and shape. Various cups, liners, shells, stems and other components may be provided in each type arthroplasty to form modular prostheses to restore function of the hip joint.
Hemi-arthroplasty refers to replacing part of a hip joint, such as replacing a femoral component so that a femoral prosthesis similar to those used in a total hip replacement articulates against natural body tissue in the patient's acetabulum. In most cases, the acetabulum is left intact and the head of the femur is replaced, using a component similar to those employed in a total hip replacement. In other cases, a hemi-surface prosthesis fits over the head of the femur so that the bone of the femoral head is spared. This hemi-surface prosthesis is then fixed to the femur with cement around the femoral head and has a short stem that passes into the femoral neck. Generally, a femoral prosthesis implanted during a hemi-arthroplasty is referred to as an endoprosthesis and includes a stem and a head, and may include additional components such as shells and liners. Current designs include monoblock, and two, three and five component designs. A monoblock endoprosthesis is a one-piece structure including a femoral stem and head. Polarity refers to the number of articulating surfaces a prosthesis contains. A monoblock endoprosthesis has one articulation surface between the head and the patient's natural acetabulum, and is therefore referred to as monopolar. Thus, a prosthesis may be described both with respect to the number of components and with respect to the number of articulating surfaces as installed in a patient. Some current designs may also include a mechanical device, such as a snap-ring, for constraining the femoral head, further described below.
Prostheses used in hip replacement surgery may also be described as constrained and non-constrained prostheses. Non-constrained prostheses rely on the downward force of the body through the joint and the tension created by the soft tissue, including the muscles, ligaments and tendons, to retain the femoral head relative to the acetabular prosthesis in its implanted position. Non-constrained prostheses generally allow the greatest range of motion. Other prostheses include mechanisms for preventing dislocation of the stem head from the acetabular component. Typically, these prostheses have restraint mechanisms that result in a smaller range of motion of the hip joint, and are generally referred to as “constrained” components. Dislocation may be the result of trauma to the hip, abnormal anatomy, soft tissue laxity, or impingement.
One example of a restraint mechanism is a shell or liner having greater than hemispherical coverage around the head such that the head is constrained within the internal diameter, thus preventing subluxation and dislocation. In contrast to standard liners, constrained liners may employ an extended, elevated portion over a segment of the periphery of the liner internal diameter in order to increase coverage of the femoral head and thus reduce the likelihood of dislocation and aid in reduction of the head should subluxation occur. While use of a constrained components is generally not desirable due to resulting decreased range of motion, the use of constrained components may be beneficial in cases of tenuous stability in order to avoid dislocation. See e.g. T. Cobb, et al., The Elevated-Rim Acetabular Liner in Total Hip Arthroplasty: Relationship to Postoperative Dislocation, Journal of Bone and Joint Surgery, Vol. 78-A, No. 1, January, 1996, pp. 80–86, which is incorporated by reference herein. However, constrained components reduce the range of motion in part because of the elevated lip segment; there is thus a substantial loss of overall range of motion compared to a standard liner. An implant stem head constrained by a shell or liner may dislocate if the femoral component rotates beyond the range of motion permitted by the assembly. Dislocation may occur because the edge or lip of the liner or shell that retains the implant stem head acts as a fulcrum about which the femoral component pivots, thereby causing the implant stem head to dislocate from its position within the liner or shell of the prosthesis. Dislocation of a hip prosthesis is painful and often requires medical intervention. Finally, a liner utilized in a constrained component must have a strong lock mechanism for retention in the cup due to the forces exerted on the liner by the other components of a constrained prosthesis. Lever out force is the moment required to dislocate the head from the liner. The ability of a prosthesis, such as an implant stem head, to withstand forces exerted on the liner is referred to as lever-out, or shuck-out. Pull out force is generally a tensile force applied in the direction of the rotational axis of the cup so as to separate the head from the cup.
During a total hip replacement procedure, the surgeon generally obtains measurements to ensure proper prosthesis selection, limb length and hip rotation. After making the incision, the surgeon works between the hip muscles to gain access to the joint. The femur is pushed out of the acetabulum and removed so that the exposed joint cavity may be cleaned and enlarged with reamers of gradually increasing size. The cup of the acetabular prosthesis is then placed in the prepared hemispherical socket. A liner may then be inserted into the cup and fixed into place. The femur is then prepared to receive the stem by reaming the center of the bone and planing and smoothing the top end of the femur. If the ball is a separate piece, the proper size is selected and attached. The assembled femoral component is then placed within the acetabular component and the joint is properly aligned. If complications such as dislocation require a surgeon to perform a revision procedure and utilize a constrained component, the surgeon may be forced to remove the acetabular component in its entirety, causing damage and bone loss.
Current constrained liner designs have many disadvantages. One design currently used does not adequately constrict movement of the components, resulting in component wear. Also, some current constrained liners are not easily removable after installation in a patient because the mechanism retaining the components is not easily accessible or reverse-operable. Other locking mechanisms are located on the exterior surface of the assembly, allowing deformation and dislocation of the locking mechanism and subsequent failure of the component. Finally, currently available designs do not offer both adequate range of motion and sufficient lever-out.
Thus, there is a need for a constrained liner assembly that offers both adequate range of motion and sufficient lever-out.
There is also a need for a constrained liner assembly that is easily disassembled after installation in a patient.
There is also a need for a constrained liner assembly that allows an existing, implanted acetabular shell to be converted to a constrained prosthesis without requiring removal of the shell and thus damage to the patient's bone and other structures.
Finally, there is a need for a liner assembly that is capable of reducing the movement of components, in order to decrease wear of the components.
These are some needs which exist in conventional designs, one, some or all of which needs are fulfilled by some or all structures of various embodiments of the invention.